Digital watermarking is a process for modifying physical or electronic media to embed a hidden machine-readable code into the media. The media may be modified such that the embedded code is imperceptible or nearly imperceptible to the user, yet may be detected through an automated detection process. Most commonly, digital watermarking is applied to media signals such as images, audio signals, and video signals. However, it may also be applied to other types of media objects, including documents (e.g., through line, word or character shifting), software, multi-dimensional graphics models, and surface textures of objects.
Digital watermarking systems typically have two primary components: an encoder that embeds the watermark in a host media signal, and a decoder that detects and reads the embedded watermark from a signal suspected of containing a watermark (a suspect signal). The encoder embeds a watermark by subtly altering the host media signal. The reading component analyzes a suspect signal to detect whether a watermark is present. In applications where the watermark encodes information, the reader extracts this information from the detected watermark.
Several particular watermarking techniques have been developed. The reader is presumed to be familiar with the literature in this field. Particular techniques for embedding and detecting imperceptible watermarks in media signals are detailed in the assignee's co-pending application Ser. No. 09/503,881 and U.S. Pat. No. 6,122,403, which are hereby incorporated by reference.
These watermarking techniques can be applied to embed auxiliary data in the compressed domain of multimedia signals. One challenge, however, is that some compression methods use a technique called “motion compensation” that may interfere with the watermark signal or create artifacts in the host signal in which the watermark is embedded. Motion compensation refers to a process often used in compression where a signal from a particular frame is predicted from one or more other frames. This technique enhances compression efficiency because the compressed version need only include a “key” frame (called the I frame), and predicted frames, coded as the difference between the frame and a predicted version of that frame.
“Drift compensation” refers to a method that prevents accumulation of visual distortions due to additive watermarking in compression schemes with motion compensation. Unless compensated properly, watermark error propagates both temporally and spatially as a result of motion compensation. Propagation of watermark signal to the consecutive frames has two potential disadvantages: it degrades the visual quality, and may interfere (most probably deconstructive interference) with the consecutive watermark signals reducing detection performance.
The invention provides a method for enhancing watermark embedding in time varying signals. One particular implementation applies to digital watermarking of compressed video stream in the MPEG compression format. However, the invention is not limited to a particular compression format. For example, the method may be applied to watermark embedding of video in the spatial domain and uncompressed video.
One aspect of the invention is a method of placing a watermark in a video stream. The method estimates motion between frames in the video stream, and computes a representative motion for a frame. Finally, the method spatially adjusts a digital watermark by the representative motion before embedding it in the frame. This method is particularly suited for embedding a watermark in a video stream compressed using motion estimation. The method can use the motion data in the video stream to compute a dominant motion for a frame, and then shift the watermark by this dominant motion before embedding it in the frame.
Further features will become apparent with reference to the following detailed description and accompanying drawings.